1. Field of the Invention
The present invention relates to an optical type head device. More particularly, it relates to an improvement in a driving current supplying means for feeding an electric current to a semiconductor laser beam source in the optical type head device.
2. Discussion of Background
There is well known an optical type head device for reading data stored on an optical type data storing medium such as an optical disc on which surface pits are formed, by supplying a laser beam and by detecting the laser beam reflected by the optical disc. Such optical type head device is used for various electric devices and appliances such as an audio PCM disc player, a video disc player and so on.
FIG. 14 shows general construction of a conventional optical type head device. The device comprises a semiconductor laser beam source 1 (hereinbelow referred to as a light source) emitting a laser beam, a beam splitter 2 for separating the emitted laser beam from the reflected laser beam, the beam splitter 2 being formed by a prism having a half-mirror portion, and a condensor lens 3 which constitutes a light converging means.
In the rear of the laser beam source 1, provided is a light detector 4 as a laser beam intensity detecting and photoelectrically converting means which outputs an electric current in proportion to the intensity of the laser beam emitted from the laser beam source 1 and which is connected to a driving power source 5 as a driving current supplying means so that an electric current is supplied to the laser beam source 1. Also, an optical disc 6 is provided at a position where the laser beam i converged by means of the condensor lens 3. A motor-driven shaft 7 is fitted in an opening at the center of the optical disc 6 so that it is rotated by a motor.
Further, a leser beam detector 9 is provided at a position where the laser beam reflected by the data storing surface 8 of the optical disc 6 is converged.
The laser beam detector 9 outputs an electric signal to a pre-amplifier 10 in which the electric current signal is converted into an electric voltage which is processed in a circuit (not shown) to be utilized for an electric device such as an audio or a video device or a digital player and so on.
The operation of the optical type head device having the above-mentioned construction will be described.
A driving current is supplied to the leaser beam source 1 by the driving power source 5 and the laser beam source 1 emitts a laser beam to the beam splitter 2. At the same time, light 12 in proportion to the intensity of the emitted laser beam 11 is generated backwardly from the laser beam source 1. The light 12 is detected by the monitoring light detector 4 which in turn, generates an electric current in proportion to the intensity of the emitted laser beam 11 to the driving power source 5 as a feedback signal. Namely, the laser beam source 1, the light detector 4 and the driving power source 5 constitute a negative feedback control loop so that a constant laser beam output can be obtained even when there is change in characteristics of the emitted laser beam to an input current.
The laser beam 11 is passed through the beam splitter 2 and is focussed on the data storing surface 8 of the optical disc 6 by means of the condensor lens. Since the data storing surface 8 has a number of pits, the laser beam focussed on the optical disc is modulated by recorded data and is reflected to the condensor lens 3. The reflected laser beam passes through the lens 3 and changes its path to the laser beam detector 9 by means of the beam splitter 2.
The laser beam detector 9 outputs an electric current depending on the intensity of the reflected laser beam 13. The electric current signal is converted into a voltage signal by a pre-amplifier 10 to be outputted from a terminal 14.
As the optical disc 6 is rotated by the shaft 7, the data on the optical disc 6 are succesively reproduced and outputted from the terminal.
The optical type head device has usually a focussing servo means for correcting focussing error of the laser beam on the optical disc and a tracking servo means for correcting deviation in the light spot of the laser beam to a data track on the optical disc although these means are omitted in the drawings.
FIG. 5 illustrates operational characteristics of the laser beam source in a relation of driving current I to the power of the emitted laser beam.
As apparent from FIG. 15, the I-P characteristics of the laser beam source 1 is in a non-linear form in which inclination of the characteristic line suddenly increses when the driving current I exceeds the threshold value Ith. Namely, the region of I&gt;Ith is a "laser oscillation region" and the region of 0&lt;I&lt;Ith is a "spontaneous emission region" in which there is no oscillation of a laser beam.
In the above-mentioned optical type head device, a coherent laser beam is converged on the optical disc to read the data on the optical disc. In this case, the driving current of the laser beam source which contributes reading of the data is only in the region of I&gt;Ith which causes laser beam oscillation.
On the other hand, an operating power P.sub.DC is typically 3 mW (Watts) in the case of the optical type head device having the I-P characteristics shown in FIG. 15. In this case, the driving current I.sub.DC is 40 mA (Ampares) and the threshold current Ith is 30 mA. Accordingly, when the laser beam source is to be driven by supplying 40 mA of the driving current I.sub.DC, only an electric current of I.sub.DC -Ith=10 A is utilized as the current to produce the laser beam necessary to operate the optical type head device.
When I&gt;Ith, change in the voltage across the PN junction (forward operating voltage) of the laser beam source is extremely small in the maximum rated current range, which can be regarded as being practically constant. Specifically, a voltage value is 1.8 V (Volts) in a device having the characteristics shown in FIG. 15.
In the conventional optical type head device, a d.c. current having a constant value is used to drive the laser beam source. As described before, since the I-P characteristics is in a non-linear form, a proportion of the threshold current Ith to the driving current I.sub.DC is large, whereby a large driving power is required to operate the laser beam source. The large driving power increases temperature at the PN junction of the laser beam source to thereby shorten the lifetime of it. There is another problem that the lifetime of a battery is shortened when it is installed in an electric device such as a portable type CD player.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical type head device which reduces a driving power from a laser beam source without deterioration of the reproduction characteristics and extends the lifetime of the laser beam source while saving consumption power.
The foregoing and the other objects of the present invention have been attained by providing an optical type head device comprising a laser beam source for emitting a laser beam, a light focussing means for converging the laser beam on a data storing surface of an optical type data storing medium, a laser beam detecting means which receives the laser beam reflected by the optical type data storing medium and modulated by data held by the same, and converts the data into electric signals, a driving current supplying means for supplying a driving current to the laser beam source, wherein the driving current supplying means comprises a high frequency current generator which supplys to the laser beam source a high frequency current having a waveform ranging from the lower part of the thresheld value at which the laser beam source starts emission of the laser beam to the higher part of the thresheld value, and the frequency of the high frequency current is at least twice as high as the maximum frequency component of information which is contained in the data storing medium.